Conventionally, ventilation members are attached to housings of electric appliances such as mobile telephones, cameras, and of automobile electrical components including lamps, pressure sensors, and ECUs (Electrical Control Units), in order to ensure ventilation between the interior and the exterior of the housings and to prevent the entry of foreign matter into the interior of the housings. Attaching such ventilation members to the housings makes it possible to prevent the entry of water, dust, and the like into the interior of the housings and at the same time to achieve the following; the pressure change inside the housing associated with temperature change can be alleviated, sound can be transmitted between the interior and the exterior of the housing, and the gas produced in the interior of the housing can be discharged to the exterior of the housing.
One example of such a ventilation member is disclosed in JP 2001-143524A (Document 1). A ventilation member 101 disclosed in Document 1 has, as shown in FIG. 16, a tubular support body 103 and a closed-bottomed protective cover 104. A gas permeable membrane 102 is disposed on one end face of the tubular support body 103. The protective cover 104 is fitted to the support body 103 so as to cover the gas permeable membrane 102. The ventilation member 101 is fixed to a housing 105 so as to cover an opening 106 of the housing 105, and gas is allowed to permeate through the gas permeable membrane 102 so that ventilation between the interior and the exterior of the housing 105 is ensured. The protective cover 104 is provided in order to prevent the breakage of the gas permeable membrane 102 caused by, for example, contact with foreign objects.
The conventional ventilation member 101, however, allows the gas inside the housing 105 and the gas outside the housing 105 to be in communication with each other via the gas permeable membrane 102 at all times, and therefore, water vapor outside the housing 105 may enter the interior of the housing 105, producing water droplets on the inner surface of the housing 105 (i.e., fogging the inner surface of the housing 105). For example, when the housing is a lamp that is one type of automobile electrical components, the water droplets (fogging) can become a cause of reducing the light intensity of the light emitted from the lamp. Such fogging is apt to occur by turning off the lamp when the air temperature is low and the humidity of the exterior the housing is high (for example, when it is raining or snowing in winter). From the viewpoint of improving the automobile's safety, there is a need for a ventilation member that can extend the time until the fogging will be produced and that allows the fogging to disappear quickly after turning on the lamp, for example, even if the fogging is produced. The same characteristics have been required for the ventilation members used for other kinds of housings.
It is possible to delay the generation of water droplets (production of fogging) by reducing the gas permeable area of the gas permeable membrane provided for the ventilation member to decrease the amount of the water vapor that enters the interior of the housing per unit time; however, in this method, the amount of the water vapor discharged to the exterior of the housing per unit time also decreases simultaneously, and therefore, it takes a long time before the generated water droplets disappear (i.e., until fogging will be cleared up). In addition, when electricity is passed through an electrical component, such as a lamp, in which the interior of the housing heats up during electricity application, when fogging has been produced therein, the water droplets are heated and water vapor is produced; however, the ventilation member provided with a gas permeable membrane having a small gas permeable area requires a long time to release the produced water vapor to the exterior of the housing.
When such a conventional ventilation member is attached to a tank (such as a bottle for chemicals, an organic solvent tank, and a gas tank), it is possible to obtain similar advantageous effects to those in the case where the ventilation member is attached to the housing, while preventing the entry of water, dust, and the like into the tank.
However, when the conventional ventilation member 101 is attached to a tank, the gas inside the tank and the gas outside the tank are in communication with each other via the gas permeable membrane 102 at all times, as in the case where the conventional ventilation member 101 is attached to the housing 105. Consequently, water vapor outside the tank may enter the interior of the tank, causing adverse effects on the liquid accommodated in the tank or causing moisture to accumulate in the tank.